The present invention relates to PTC thermistor elements used in switching elements for example. and methods for producing the same.
Barium tltanate-type ceramics have been widely used as a material for ceramic filters and capacitors because of the high resistivity and dielectric constant. Also, barium titanate-type semiconductors, which contains a small amount of rare earth elements or the like as a dopant, have been used in temperature sensor elements and switching elements as a positive temperature coefficient (PTC) thermistor due to their PTC characteristics or the characteristic that the electric resistance thereof undergoes a sudden change around the Curie point where a phase transition occurs between the ferroelectric phase and the paraelectric phase.
In recent years, there has a proposal of an application of semiconductor film in the PTC thermistor element, in place of a bulk or sintered block of the semiconductor in a conventional PTC thermistor element. Such application of the thin PTC thermistor film enables the element a decrease in heat capacity thereof and Improvement in response characteristic. Further, such application of film could improve the quality and reliability of small-sized PTC elements since control in thickness of the film in producing process can be better exercised than in fine working of the condenser bulk. Another advantage is that the film is excellent in mechanical strength.
The film PTC thermistor is opening up various new possibilities including monolithic incorporation of thermistor elements on an Integrated circuit.
A typical PTC thermistor element using the film is shown in FIG. 10.
As shown in FIG. 10, the film PTC thermistor element has an insulating substrate 53, a PTC thermistor film 52 of barium titanate-type semiconductor formed thereon, and a pair of comb-shaped electrodes 51a and 51b made of aluminum films formed on the PTC thermistor film 52.
The barium titanate-type semiconductor film is generally formed by the sputtering method. The sputtering technique can control a quantity of rare earth elements such as Sr, Mn and Y to be added to impart PTC characteristlcs to the film better than the vacuum deposition method or the chemical vapor deposition (CVD) method.
An addition of Sr, Sn, Zr or Pb changes the temperature at which the film sharply changes in resistance An addition of Y, Nb, Ta, Si, Sb, W. La, Ce, Pr, Nd, Sm, Gd or Ho makes the film semiconductive up to the inside of the crystal grain An addition of Mn, Fe, Cu or Cr Increases the change ratio in electric resistance with temperature rise. An addition of Si makes crystal grains in the film uniform in diameter
The PTC thermistor element is produced as the following process, for example.
First, the PTC thermistor film 52 is formed on the alumina substrate 53 by the sputtering technique, followed by heat treatment for three hours at 1.000xc2x0 C.
Then, the comb-shaped electrodes 51a and 51b is formed on the surface of thus formed PTC thermistor film 52. Here, single base metals such as Al, Ni, Cu, Au, In, Ag, Hg, Ga or Zn and their alloys, which can have an ohmic contact with the PTC thermistor film 52, are employed as the materials of the comb-shaped electrodes 51a and 51b.
Those electrodes 51a and 51b are formed by such techniques as vacuum deposition, printing, plating and sputtering.
If noble metals such Pt excellent in high-temperature durability are used in place of the aforesaid base metals, a shottky barrier which sends up the apparent electric resistance of the element will be inevitably formed and no ohmic contact will be produced between the PTC thermistor film and electrode.
There is a growing demand for film PTC thermistor elements. But such elements are too high in electric resistance to build in the integrated circuit.
The electric resistance of PTC thermistor element at room temperature is decreased when the thickness of the PTC thermistor film is increased. But, a PTC thermistor film as thick as tens of microns formed on a substrate would peel off from the substrate because of an internal stress thereof. That is, a firmly formed film would have a thickness of only several microns.
As a solution to that problem, a PTC thermistor element as shown in FIG. 1 has been proposed. This thermistor element has an electrode 2 formed on the surface of an insulating substrate 1, and a PTC thermistor film 3 and the other electrode 4 formed thereon. It is hoped that adopting such a lamination structure In which a PTC thermistor film is sandwiched between a pair of electrodes could lower the electric resistance of the element.
However, the PTC thermistor element of that lamination structure falls to exhibit fully desired characteristics on the following ground.
That is, to form a PTC thermistor film by the sputtering technique, it is necessary to raise the substrate temperature in forming a film and the subsequent heat treatment temperature up to 800xc2x0 C. or higher as described in xe2x80x9cElectronic Ceramicsxe2x80x9d (Sep. 1987, p. 28 to 33) and Japanese Laid-Open Patent Publication No. Hei 2-77102 . The electrode 2 to be formed before the PTC thermistor film 3 is exposed to that high temperature for one hour or more. Base metals used as the electrode material are oxidized at that high temperature, losing the dielectric constant or diffusing into the PTC thermistor film and deteriorating the PTC characteristics of the formed film.
An object of the present invention is to solve the above-mentioned problems thereby to provide a PTC thermistor element with excellent PTC thermistor characteristics.
The method for making a PTC thermistor element of the present invention comprises the steps of: forming a first electrode on an insulating substrate; forming a PTC thermistor film containing barium titanate as a main component on the first electrode; heat-treating the substrate; and forming a second electrode on the film, wherein the heat treatment of the substrate is rapid heating using a heat irradiation.
In making a PTC thermistor element, a first electrode, PTC thermistor film and second electrode are formed on the substrate in this order. Therefore, it is observed that the material contained in the first electrode diffuses into the PTC thermistor film at forming the PTC thermistor film on the first electrode, or at annealing the formed PTC thermistor film. As a result, the PTC thermistor film fails to develop a perovskite crystal structure and to exhibit the desired characteristics.
The diffusion of the electrode material depends on the temperature and time of the heat treatment of the PTC thermistor film. Therefore, the diffusion into the PTC thermistor of the electrode material can be kept down when the PTC thermistor is heated rapidly and then cooled in a short time thereby to shorten the time in which the electrode is exposed to a high temperature.
Preferably, the maximum temperature of the substrate in the heat treatment is set in a range of 900 to 1,500xc2x0 C., and the time of treatment including the heating and cooling is set at 0.5 to 5 minutes. The preferable treatment time depends on the diffusion rate of the electrode material at the maximum temperature. If the heating In one session of the above treatment is not enough for developing the desired PTC thermistor characteristics, the heating under the same conditions may be repeated a number of times For example, in a case of Ni as the electrode material, one heat treatment for 300 seconds is enough at 900xc2x0 C., but a repetition of five times of the heat treatment for 30 seconds is desired at 1,500xc2x0 C. With Al, it is desirable to conduct the heat treatment for 30 seconds at 900xc2x0 C. for 20 times.
It is desirable to use a lamp heater for the rapid heating of the substrate.
A PTC thermistor element of the present invention includes: a barium titanate-containing PTC thermistor film; an n-type semiconductor connected to the PTC thermistor film; a first electrode connected to the PTC thermistor film; and a second electrode connected to the n-type semiconductor.
In an example of making the PTC thermistor element, the PTC thermistor film is formed on a surface of a substrate made of n-type semiconductor, and a pair of the electrodes are then formed on each side of the substrate so as to sandwich the substrate and the PTC thermistor film formed thereon therebetween. The pair of electrodes may also be formed on the same side. In this case, an insulating substrate provided with an n-type semiconductor film on the surface may be used in place of the substrate of n-type semiconductor.
An n-type semiconductor placed between the PTC thermistor film and the electrodes would work as a part of the electrode. Even if, therefore, valiances are observed in workmanship and electric characteristics, the PTC thermistor characteristics of the element are hardly influenced as a whole. Further, no shottky barrier will be formed between the PTC thermistor film and the n-type semiconductor.
The n-type semiconductor may be a semiconductor formed with the barium titanate of the perovskite-type crystal structure as a main component. Alternatives may be semiconductors with the main component being Si, zinc oxide, titanium oxide, iron oxide, tin oxide, etc.
Under the above-mentioned arrangement, electrodes can be formed after a PTC thermistor film being formed. Therefore, the heat treatment of the PTC thermistor film can be performed at a high temperature and a film of a higher quality can be obtained. Besides, it is possible to prevent the electrode material from diffusing into the PTC thermistor film. Thus, the present invention can provide a PTC thermistor element which is low in electric resistance at room temperature.
The suitable electrode materials include aluminum, nickel, zinc, copper, silver, Inxe2x80x94Ga and Inxe2x80x94Hg. Another PTC thermistor element of the present invention has a plurality of PTC thermistor units, each provided with a PTC thermistor film containing barium titanate as well as a pair of electrodes, all the PTC thermistor units parallel-connected to each other, with at least one of them connected In reverse manner.
If a noble metal is used for the electrode, a shottky barrier will be formed between the PTC thermistor film and the electrode, exhibiting diode-like rectification characteristics. Therefore, when a plurality of PTC thermistor units are parallel-connected to each other and at least one of them is connected to the others in reverse manner, the rectification characteristics can be offset, and thus a PTC thermistor element with a low electric resistance at room temperature can be obtained. Therefore, it becomes possible to use noble metals as electrode materialsxe2x80x94such noble metals as Pt, Ru, Rh, Pd, W, Re, Os, Ir and Au.
The above-mentioned PTC thermistor unit is of such a lamination structure that the PTC thermistor film is sandwiched between a pair of electrodes. It may also be so arranged that one of the electrodes and the PTC thermistor film are placed on the p-type semiconductor layer formed on the same side of the substrate while the other electrode is put on the PTC thermistor film. Another arrangement is that one electrode is provided on a side of a substrate made of a p-type semiconductor and a PTC thermistor film with another electrode provided thereon is provided on an opposite side of the substrate. That is, the both electrodes are arranged so as to face each other via the substrate.
Of the electrodes, the one that 1s formed after the formation of the PTC thermistor film may be made of aluminum, nickel, Inxe2x80x94Hg alloys and Inxe2x80x94Ga alloys.
The aforesaid p-type semiconductor may be semiconductors formed with the main component being silicon, nickel oxide, cobalt oxide, iron oxide, manganese oxide, bismuth oxide or chromium oxide.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings